Once your map gets big enough, you get as ABS said, Map Max plane error (hit it today, damn shame). You also have to consider how big the maps would be, in mb. Cranefight? not that big, it's 17mb big. Lockdown is 10mb, so unless they get better compiling tools (Oh, I hope so, it's nessesary) Ns maps themsevles would easily be bigger then 20, 30 mb. 30mb per map, well damn, that will be a huge ass download, and downloading custom maps from the server is practically out of the question.
EDIT: Mind you, once cranefight was zipped, it went form 17mb to 5mb, So theres deffinently improvment for compiling, Im sure